Olefin polymers are shaped by various methods and used in wide-ranged applications. For example, ethylene polymers are extruded into films or sheets for use in the packaging of foods, liquids or daily sundries. Olefin polymers require various properties depending on the shaping methods or use applications. In the case of T-die extrusion as an example, they require performances such as stable processability even at high speed (high-speed film-forming properties) and small neck-in.
Low density polyethylenes (LDPE) by high-pressure radical polymerization have a high melt tension because of their complicated long-chain branched structures, and show good shaping processability such as small neck-in, thereby finding various uses. However, shaped articles therefrom still have low mechanical strength properties such as tensile strength, tear strength and impact resistant strength. Further, these polymers show poor high-speed film-forming properties in T-die extrusion.
In contrast to LDPE, Ziegler-catalyzed or metallocene-catalyzed ethylene polymers possess high tensile strength, tear strength and impact resistant strength due to their molecular structures, and they are used in applications requiring mechanical strength. However, these polymers have a low melt tension and consequent poor shaping processability.
To solve these problems, [1] LDPE is blended with a Ziegler-catalyzed or metallocene-catalyzed ethylene polymer (Patent Document 1); [2] the molecular weight distribution is broadened by multistage polymerization (Patent Document 2); [3] a long-chain branched ethylene polymer is produced with a chromium catalyst; [4] a long-chain branched ethylene polymer is produced with a specific metallocene catalyst (Patent Document 3); [5] macro monomers are copolymerized with use of a specific metallocene catalyst to give a long-chain branched ethylene polymer (Patent Document 4); or [6] ethylene and diene are copolymerized with use of a specific metallocene catalyst to afford a long-chain branched ethylene polymer (Patent Documents 6 and 7). However, the method [1] greatly increases costs in the blending of the polymers, and the ethylene polymers obtained by the methods [2], [3], [4] and [5] have a small number of long-chain branches and do not have a sufficient melt tension or shaping processability. Further, the method [6] can deteriorate mechanical characteristics inherent to polymers or can result in gelation when the diene is used in large amounts.
Patent Documents 8 and 9 teach the use of two or more kinds of metallocene compounds or organometallic complexes in order to produce more long-chain branches or to increase the melt tension. However, the number of long-chain branches is still insufficient and problems remain in terms of shaping processability. Further, the catalytic activity is far below the industrial level.
As discussed above, it has been difficult to produce resins having high melt tension and excellent mechanical strength inexpensively and efficiently by means of the conventional catalyst systems or by blending resins. In other words, the development of efficient production processes for ethylene polymers having high melt tension and excellent mechanical strength is important and highly valuable in the industrial production.
When ethylene polymers are used as sealants in packaging materials, the polymers require mechanical strength such as heat seal strength or pack breakage strength to protect the contents. However, packaging materials that are easily opened (have easy openability) attract attention out of consideration for elderly people, infants and disabled people. One of the approaches for easy openability is to appropriately weaken the heat seal strength at the sealed portion. Accordingly, there is a need for ethylene polymers having appropriately low heat seal strength.
The present inventors studied diligently in view of the problems in the art as describe above. They have then found that a single or plural kinds of bridged metallocene compounds having differing cyclopentadienyl-derived groups can afford macromonomers that are a source of long-chain branches and can simultaneously catalyze the repolymerization of the macromonomers into olefin polymers having a large number of long-chain branches, small neck-in in the T-die extrusion, small take-up surge and superior mechanical strength or olefin polymers having small neck-in in the T-die extrusion, small take-up surge and easy opening properties. Such compounds as olefin polymerization catalysts and polymerization processes using the compounds have been found to be capable of efficiently producing the olefin polymers as described above. The present invention has been completed based on the findings.    Patent Document 1: WO 99/046325    Patent Document 2: JP-A-H02-53811    Patent Document 3: JP-A-H04-213306    Patent Document 4: JP-A-H08-502303    Patent Document 5: JP-A-H04-213306    Patent Document 6: JP-A-H09-227626    Patent Document 7: JP-A-H04-506372    Patent Document 8: JP-A-H07-252311    Patent Document 9: JP-A-2006-2057